Calculating device

ABSTRACT

A device for calculating the thickness of a blank which is required to make an optical lens of specified characteristics. The device provides for the specification of the front and back surface powers of the lens, the minimum thickness allowable in the finished lens, the effect of prism and the correction in the effective power due to the center thickness of a convex lens, each for a specified diameter of lens. The device may take either of two forms, a linear sliding calculator or a rotary disc calculator.

United States Patent Eadon-Allen 51 Sept. 12, 1972 CALCULATING DEVICE [72] inventor: Stuart Eadon-Allen, Birmingham,

England [73] Assignee: Dollond and Aitchison Services Ltd.,

Birmingham, England [22] Filed: Oct. 6, 1971 [21] Appl,No.: 187,019

[52] US. Cl. ..235/70 R, 235/78 [5 1] Int. Cl. ..G06g l/02 [58] Field of Search.....235/70 R, 70 A, 78, 88, 85 R,

[56] References Cited UNITED STATES PATENTS 2,696,757 12/1954 Richards ..235/61 B 3,163,358 12/1964 Grolman ..235/89R Primary Examiner-Richard B. Wilkinson Assistant Examiner-Stanley A. Wal Attorney-Merriam Marshall, Shapiro & Klose [5 7] ABSTRACT A device for calculating the thickness of a blank which is required to make an optical lens of specified characteristics. The device provides for the specification of the front and back surface powers of the lens, the minimum thickness allowable in the finished lens, the effect of prism and the correction in the effective power due to the center thickness of a convex lens, each for a specified diameter of lens. The device may take either of two forms, a linear sliding calculator or a rotary disc calculator.

12 Claims, 2 Drawing Figures PATENTEDsmwn 3.690.547

summrz Fla. 1.

PATENTED SEP 12 I972 SHEET 2 OF 2 CALCULATING DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention concerns a new calculating device which is designed specifically for use in the optical industry in connection with the manufacture of lenses.

The invention has been developed primarily in relation to the manufacture of spectacle lenses, but it will be apparent that it is also applicable to the manufacture of lenses generally. v

2. Description of the Prior Art Spectacle lenses are normally manufactured by grinding an appropriately dimensioned optical glass blank to producethe required curvatures on opposite sides thereof.

Such blanks are normally available in a wide range of thicknesses and diameters and the choice of the correct blank to form the starting point for any'particular lens normally requires the performance of a series ofcalculations, or at least the inspection of several tables, unless the lens is a standard one which is produced frequently so that the appropriate blank is readily known.

It is not difficult to determine the diameter of the blank which should be employed for the production of any particular lens, since it is only necessary to compare the blank with the spectacle frame in order to ascertain whether it is large enough. Similarly, where the blanks are not of a full circular shape but are partially pre-shaped in outline, there is little difficulty in selecting the appropriate shape.

However, the main difficulty arises in determining the correct thickness of the blank. It will be understood that if the selected blank is too thin, the grinding of the surfaces to the required curvature will lead to said survaces intersecting within the circumference of the blank. Thus, in the preparation of a convex lens, the thickness of the blank would diminish to zero at the edge of the lens and thus reduce the diameter of the lens so that it would be unusable. Alternatively, if the lens were concave, the thickness of the blank would be reduced to 'zero in the center of the blank and again the lens would be unusable. I

On the other hand, if the chosen blank is thicker than necessary, this will give rise to the production of an unnecessarily heavy lens, or additional material will have to be ground away in order to reduce the lens to the required thickness. This clearly represents a waste of valuable optical glass and also results in an increase in the time taken to produce the lens.

BRIEF SUMMARY OF THE INVENTION The principal object of the present invention is to provide a calculating device which enables the optimum thickness for a blank to be determined without calculation.

According to the present invention we provide a calculatin g device for determining the thickness of a blank required to produce a lens, the device comprising surface power scale means graduated in teims of lens surface powers;

surface power indicator means for indicating a selected value of lens power for the front and for the back surface of the lens;

thickness scale means graduated in terms of thickness and movable relative to said first scale means; and

thickness indicator means for indicating values of thickness on the thickness scale means; means being operatively associated with the surface power scale means and the thickness indicator means for ensuring that, for selected values of front and back surface power for the lens, the appropriate thickness of the blank is indicated on the thickness scale means by the thickness indicator means.

The surface power scale means may comprise a first scale graduated in terms of front surface powers, a second scale movable relatively to the first scale and graduated in terms of back surface powers, so that any chosen value of back surface power can be aligned with any chosen value of front surface power, and the thickness scale means may comprise a third scale disposed adjacent to the second scale and graduated in terms of lens thickness and fixed in position relative to said first scale, read-out marker means being provided in fixed position relative to the second scale for indicating on the third scale the appropriate thickness of the blank, when a selected value of front surface power on the first scale is aligned with a selected value of back surface power on the second scale.

Thus, with the device in accordance with the invention it is merely necessary to align the desired front and back surface powers on the first and second scales in order to obtain direct reading of the appropriate blank thickness from the third sale.

Preferably, the first and second scales are each divided into a plurality of sub scales which extend parallel to one another and are graduated in terms of surface powers in relation to blanks of different diameters.

It will be understood that, in general, the appropriate blank thickness will vary according to the chosen diameter of the blank for any given combination of front and back surface powers.

It is particularly convenient for the first and second scales to have minimum graduations corresponding to the minimum increments in which such powers are normally expressed, for example one-fourth Dioptre, and all three scales are preferably so graduated and arranged relative to one another that alignment of any chosen front and back surface powers on the first and second scales gives an indication on the third scale which corresponds exactly to a graduation marking.

This arrangement has the advantage that the operator is not required to select one of two possible values which would otherwise be the case if the read out marker indicated a value between two graduation markings on the third scale.

Preferably, these scales are so arranged that the value indicated on the third scale includes a pre-determined allowance for the minimum thickness of the lens to be produced from the blank. For example, it is in general undesirable for a finished concave lens to have a center thickness of less than 1 mm, or a finished convex lens to have an edge thickness of less than 1 mm, and this allowance can be automatically provided for in the alignment of the scales.

Further, the surface power indicator means and thickness indicator means may comprise a cursor which is slidable relative to all the scales and includes a reference line extending transversely thereto so as to facilitate the alignment of selected values of front and back surface powers on the first and second scales and to facilitate reading of the third scale when the cursor is appropriately positioned.

Additionally, the cursor may carry additional read out markers spaced from the reference line by distances which correspond to an appropriate finishing allowance which would necessitate the use of a blank having a thickness greater .than that indicated by the primary read out marker. For example, two such further read out markers may be provided and so spaced from the reference line of cursor that they indicate respectively a semi-finished andrough finished allowance. The third scale can then thus give a reading of the appropriate blank thickness either without any finishing allowance or with a semi-finish or rough finish allowance as appropriate in any given circumstance.

Preferably, there is associated with the second scale, and fixed in position relative thereto, a fourth scale graduated in terms of prism compensation. Conveniently, the primary read out marker serves as the zero point of this prism compensation scale which is so graduated relative to the third scale as to provide for a reading on the third scale increased by the extent required by a selected prism factor on the fourth scale.

Again, if necessary the reference line of the cursor can be aligned with any selected value of prism factor on the prism compensation scale after the appropriate front and back surface powers have been aligned on the first and second scales so that the reference line of the cursor then indicates the appropriately increased value on the third scale. Of course, the further allowance provided for by the additional read out markers on the cursor can still be applied.

One further problem which arises duringv the manufacture of lenses, is that in some cases the nominal front surface power requires a correction to compensate for the effect of the thickness of the lens, it being necessary to reduce the front surface powder in order to produce a lens of the desired optical strength The correction factor required is difficult to calculate, but since it is related to the blank thickness it is convenient to incorporate in the calculating device in accordance with the invention supplementary scales to determine this correction factor.

Two supplementary scales may be provided, each being graduated identically in terms of front surface power, one of these supplementary scales being fixed in position relative to the first scale and the other supplementary scale being slidable relative to the first scale, the arrangement being such that the slidable supplementary scale can be set in a position which is determined by the relative positions of the first and second scales in such a way that the corrected value of front surface power appears on the fixed supplementary scale against the selected value of the front surface power as indicated on the slidable supplementary scale.

Conveniently, the slidable supplementary scale is mechanically connected with the second scale in such a way that movement of the second scale automatically moves the slidable supplementary scale so that the corrected value of front surface power appears on the fixed scale against the selected value of front surface power as indicated on the slidable scale.

Alternatively, the slidable supplementary scale may include an indicating marker which is disposed adjacent an additional scale which is fixed in position relative to the first scale, such additional scale being graduated in terms of blank thickness so that the indicating marker can be set in a position in which it is aligned with the same value of blank thickness on the additional scale as that which is indicated by the readout marker on the third scale.

The calculating device according to the invention may alternatively have surface power scale means comprising a disc having at least one scale thereon graduated in terms of lens surface powers and surface power in dictator means comprising a pair of cursors rotatable relative to the disc about the axis of the disc, each cursor being capable of indicating a value of lens power on the or each scale on the disc.

In this form of device, the further scale means graduated in terms of thickness and movable relative to the first scale means comprises a second disc mounted coaxially with the first disc for rotation relative thereto and the indicator means for the further scale may comprise one of the two cursors.

A plurality of scales may be provided on the first disc, each scale being graduated in terms of surface powers and being of part-circular form, the scales being concentric with the disc and each scale relating to a blank of a different diameter.

A scale graduated in terms of prism compensation may be associated with the or each scale on the first disc in such a way that setting ofone of the pair of cursors to a selected value of prism compensation causes the thickness of the blank indicated on the further scale means to be corrected by the extent required by the selected prism factor.

A supplementary scale graduated in terms of front surface power and comprising a'third disc mounted coaxially with said first and seconddiscs may be provided, an indicating pointer being provided for the third disc and said pointer being moved proportionately to the movement of one of the said pair of cursors, the arrangement being such that the nominal front surface power, as indicated on the third disc can be set in position relative to the other components of the device in such a way that, when the thickness of the blank is indicated on the further scale means, the compensated front surface power is indicated by the indicating pointer on the third scale.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings in which,

FIG. 1 shows a partial plan view of one embodiment of calculating device in accordance therewith, and

FIG. 2 shows an exploded perspective view of a second embodiment of calculating device in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION As shown in FIG. 1 of the drawings, a first embodiment of calculating device is shown which includes a main board or base 10 which is provided with two lonrespectively.

gitudinally extending generally rectangular section grooves 11 and 12. Members 13 and 14 are respectively slidable in the grooves 11 and 12. The base further includes a shoulder 15 which forms a running surface for one end of a cursor 16, the other end of 5 which runs on the upper edge of the base 10 as shown.

The scale A is sub-divided into three parallel subscales A A and A Each scale is graduated in terms of front surface powers expressed in Dioptres, the graduation being disposed at intervals corresponding to one-fourth Dioptre. The respective sub-scales are so graduated as to relate to differing diameters'of blank. For example, sub-scale A may be graduated as appropriate for a blank having a diameter of 60mm whereas sub-scales A and A may be graduated as appropriate for blanks having diameters of 56 and-52 mm Further sub-scales may be provided if necessary to allow for blanks of other sizes, or the equivalent information could be expressedin a graphical form although this would be less convenient in practice.

The movable member 13 carries scale B which is sub-divided into sub-scales B B and B are shown. These scales correspond to the scales A,, A and A and are likewise graduated in terms of back surface power expressed in Dioptres with graduations corresponding to one-fourth Dioptre.

The scale C is graduated in units of 0.1 mm and indicates the appropriate thickness of blank.

Touse the device, the required front and back surface powers are aligned on the sub-scales appropriate to the diameter of blank necessary with the aid of a reference line 17 on the cursor 16.

Therequired thickness of blank is then indicated by a primary reference marker l8 on the slidable member 13 adjacent to the scale C. The appropriate thickness of blank can thus be read off directly from the scale C.

However, the cursor 16 carries two further read out markers 19 and which are spaced from the reference line 17 by distances corresponding to the amount by which the blank thickness would have to be increased in order to provide a finishing allowance of a specified degree. Thus the markers 19 and 20 provide for a semi-finished and a rough finished allowance respectively and indicate on the scale C the appropriately increased thickness of the required blank.

The primary read out marker 18 also forms the zero point for a scale D which is graduated in terms of prism correction. Where a lens requires a degree of prism, this fact must be allowed for in selecting the blank which must be of an appropriately increased thickness.

Thus, when the lens under consideration includes a prism factor, the cursor 16 can be set so that the reference line 17 registers with the appropriate prism correction factor on scale D, the required blank thickness then being indicated by the reference line 17 where it crosses the scale C. Of course, the finishing allowance provided for by the markers 19 and 20 can be added to this value if necessary. It will be understood that where there is no prism correction factor required, the reference line 17 of the cursor 16 would normally be aligned with the primary reference marker 18 in order to facilitate the reading of the scale C so that the further read out markers 19 and 20 would be positioned as appropriate to provide an indication of the finishing allowances.

In order to calculate the corrected front surface power in a case where such correction is necessary due to the thickness of the lens, the further scales E and F are provided. Each scale is identical and is graduated in terms of front surface power expressed in Dioptres.

In a preferred arrangement, the members 13 and 14 are so coupled that the member 14 moves in the op posite direction to the member 13 through a proportionately reduced distance so that the scales E and F are displaced relative to one another.

In order to read out the corrected front surface power, it is merely necessary to note the value of front surface power which appears on the scale E in alignment with the nominal front surface power as represented on the scale F. The scale F is automatically displaced to the appropriate extent to provide the corrected front surface power by virtue of its linkage with the member 13 which carries the scale B.

Alternatively, the members 13 and 14 may be movable independently of each other and in this case, the member 14 would include a reference marker 21 which registers with an additional scale C which is graduated in terms of blank thickness but on a reduced scale as compared with scale C. In this case, the reference marker 21 would be aligned with the graduation mark on scale C which represents the value of blank thickness read out from scale C.

Scale C is preferably so arranged that the value of blank thickness indicated thereon by the primary read out marker 18, or the reference line 17 of the cursor 16, includes an allowance for a predetermined minimum lens thickness. For example, the scale may provide an indication of the appropriate blank thickness on the assumption that the minimum thickness of the lens is to be 1 mm. Such minimum thickness, of course, occurs in the center of a concave lens and at the edge of a convex lens.

Although the first embodiment of the device described above .is in linear form, it will be understood that it can alternatively be constructed in a circular form, and FIG. 2 of the drawings shows a second embodiment of the device which is so constructed.

Referring now to FIG. 2 of the drawings, it will be seen that the second embodiment of the device includes a number of discs which are mounted co-axially in relation to each other on a mounting plate 110. A spindle 111 passes through the centers of the discs and also passes through the mounting plate 1 10.

A knob 112 is fixed at one end of the spindle 111, while the other end of the spindle terminates gear train 113, which is used for a purpose to be described later in the specification.

A main cursor 114 comprising a circular disc of transparent material bearing a fine reference line 115, is secured to a boss 116 which is splined to the spindle 111 at a position near the knob 112.

A secondary cursor 117 in the form of a pointer is also mounted on the boss 116, but is freely rotatable in relation to the boss. A washer 118 and a coil spring 119 are interposed between the secondary cursor 117 and the knob 112 and the spring 119 urges the secondary cursor 117 into abutment with an annular shoulder 120 surrounding the boss 116, so that the secondary cursor 1l7 normally rotates with the main cursor 114 but can be manually moved in relation to the main cursor by overcoming the frictional resistance due to the spring loading.

Considering the portion of the spindle adjacent the main cursor 114 and on the opposite side of the main cursor from the knob 112, the spindle next passes through a main scale disc 121 which carries a plurality of scales which will later be described in more detail. The main scale disc 121 is not keyed to the spindle'but is mounted on the mounting plate 110 by means of three pins which pass through three circumferentially spaced holes in the central portion of the. main scale disc which pass through corresponding holes in an adjacent washer 122.

The outer periphery of the washer 122 abuts against the face of a thickness scale disc 123 which carries a scale of thickness to be described later in the specification and which is freely rotatable in relation to the spindle ll.

On the opposite side of the thickness scale disc 123 from the washer 122 there is a fixed spacer plate 124 which is also mounted, by means of the three pins, referred to above, on the mounting plate 110. The spacer plate is in the form of a circular disc having a sector removed therefrom leaving a space which is generally indicated at 125 in the drawing. A pointer 126 is disposed within the space 125 and is capable of a limited degree of movement from side to side within the space 125. The pointer 126 is mounted on a sleeve 127 which surrounds the spindle 111 and extends from the pointer 126 in a direction away from the knob 112.

The spindle 111 and sleeve 127 next pass through a further washer 128 which is similar to the washer 122 except that the central hole of the washer 128 is larger than that of the washer 122, to enable the sleeve 127 to pass through the hole. The washer 128 is provided with three holes to enable the three pins secured to the mounting plate to pass through.

The rear face of the washer 128 abuts against the frontface of a compensated surface power scale disc 129 which carries a scale which will be described in more detail below, the compensated surface power scale disc 129 being freely rotatable in relation to the spindle 111 and sleeve 127 which pass therethrough.

After passing through the compensated surface power scale disc 129, the spindle 111 and sleeve 127 next pass through an aperture in the fixed mounting plate 10 to the gear train indicated at 113.

The sleeve 127 is rotatable with respect to the spindle 111 and, as will be seen with reference to FIG. 2 of the drawings, the gear train causes the sleeve 127 to rotate in a'direction opposite to the direction of rotation of the spindle 111, the amounts of rotation of the spindle and sleeve being proportional.

Thus, when the main cursor 114 is rotated a certain amount in a clockwise direction, for example, the pointer 126 which is mounted on the sleeve 127 is rotated by a proportionate amount in an anti-clockwise direction.

The main scale disc 121 carries a plurality of scales calibrated in terms of optical power, in units of onequarter Dioptre. A plurality of scales are provided, the scales being graduated so as to relate to differing diameters of blank. The reference line 115 of the main cursor 114 can be used in relation to each of the scales to indicate a chosen value of power. Similarly, the secondary cursor 117, which may also be made of transparent material and have a reference line marked thereon, can be used to indicate a value of power on any of the scales of the main scale disc 121.

The thickness scale disc 123 is of larger diameter than the main scale disc 121 and the edge of the thickness scale disc 123 therefore projects outwardly beyond the edge of the main scale 121. A scale of thickness is marked around the outer periphery of the thickness scale disc 123 and this is visible surrounding the main scale disc 121 when viewed from the knob end of the spindle Ill. The reference lines of the main cursor 114 and secondary cursor 117 can be used to indicate particular values on the scale disc 123.

The device can be used in performing calculations of thickness for bi-convex lens forms, bi-concave lens forms and curved-lens forms of positive or negative overall optical powers.

The operation of the device will firstly be described in relation to calculating the edge thickness of bi-concave lens having specified back surface power and front surface power and a required minimum center thickness.

The appropriate power scale is selected on the main scale disc 121, according to the overall diameter of the lens which is required.

The reference line 115 of the main cursor 114 is set at the zero of the appropriate scale and held in position manually whilst the secondary cursor 117 is rotated in relation to the main cursor against the pressure of the spring 119, until the secondary cursor indicates the required back surface power on the chosen power scale.

The main cursor is then swung by means of the knob 112 and carries the secondary cursor with it, ina fixed angular relation to the reference line 115 of the main cursor depending upon the back surface power require for the lens, and the main cursor is moved until the secondary cursor indicates the zero of the scale.

The thickness scale disc 123 is then rotated relative to the spindle 111 until the required center thickness of the lens is indicated against the reference line 115 on the main cursor.

The main cursor is then swung to indicate the required front surface power on the main scale disc 121, care being taken that the same power scale is selected on the main scale disc, according to the overall diameter of the lens which is required, and the position of the main cursor against the thickness scale disc 123 then indicates the appropriate thickness of the blank required for making the lens.

As was the case in the previously described embodiment shown in the drawings accompanying the provisional specification, it is possible to compensate for the effective thickness in the lens, which will effect the power of the lens. This effect is most significant in convex lenses, since these have a greater center thickness than edge thickness. In order to compensate for lens thickness, the compensated surface power scale disc 129 is used.

This can best be illustrated by taking the example of calculating the center thickness of a bi-convex lens, together with the compensated front surface power, and this calculation is performed as follows.

The main cursor 114 is set to the zero of the appropriate power scale on the main scale disc 121, according to the required diameter of the lens. The secondary cursor 117 is set to the back surface power which is required for the bi-convex lens, without moving the main cursor 114.

The thickness scale 123 is then rotated relative to the main scale disc 121 in order to set the required minimum edge thickness against the reference line 115 of the main cursor.

The compensated surface power scale disc 129 is.

scale disc 129. The main cursor is then slowly moved back, causing the pointer 126 to move back in the opposite direction and the movement of the main cursor 114 is stopped when the two pointers indicate the same value of power on the scales 121 and 129. This forms a check on the accuracy of setting of the pointer 126 against the compensated surface power scale disc 129.

The main cursor 114 is then swung to the nominal front surface power on the main power scale 121. The center thickness of the lens blank is then indicated by the position of the reference line 115 against the thickness scale 123, whilst the compensated front surface power is indicated by the position of the pointer 126 on the compensated power scale disc 129.

The main cursor 114 is then swung to the nominal front surface power on the main power scale 121. The center thickness of the lens blank is then indicated by the position of the reference line 115 against the thickness scale 123, whilst the compensated front surface power is indicated by the position of the pointer 126 on the compensated surface power scale disc 129.

The compensated front surface power can also be included in calculations for curved lens forms which have an overall positive power, since again in this case the center thickness of the lens will be larger than the edge thickness and may have a significant effect on the effective power of the lens. The effect of prism on the lens center thickness may be calculated using the-main power scale 121, which also includes prism corrections on each of its scales. In the case of the bi-convex and bi-concave lens calculations which are described above, the main cursor is first of all set to zero on the power scale and the secondary cursor is then set to the back surface power. Then the main cursor is swung so that the secondary cursor, which is carried by the main cursor is set to the zero. At this stage, the prism correction can be made by holding the main cursor in position and moving the secondary cursor from the zero position on the scale to the prism setting required, whereupon the secondary cursor is then returned to the zero setting by swinging the main and secondary cursors together. The remainder of the bi-concave or biconvex lens calculation is then continued as set out above.

What we claimthen is:

1. A calculating device for determining the thickness of a blank required to produce a lens, the device comprising surface power scale means graduated in terms of lens surface powers surface power indicator means for indicating a selected value of lens power for the front and for the back surface of the lens thickness scale means graduated in terms of thickness and movable relative to said first scale means; and

thickness indicator means for indicating values of thickness of the thickness scale means means being operatively associated with the surface power scale means and the thickness indicator means for ensuring that, for selected values of front and back surface power for the lens, the appropriate thickness of the blank is indicated on the thickness scale means by the thickness indicator means.

2. A calculating device according to claim 1, wherein the surface power scale means comprises a'first scale graduated n terms of front surface powers, a second scale movable relatively to the first scale and graduated in terms of back surface powers, so that any chosen value of back surface power can be aligned with any chosen value of front surface power, and wherein said thickness scale means comprises a third scale disposed adjacent to the second scale and graduated in terms of lens thickness and fixed in position relative to said first scale, read-out marker means being provided in fixed position relative to the second scale for indicating on the third scale the appropriate thickness of the blank, when a selected value of front surface power on the first scale is aligned with a selected value of back surface power on the second scale.

3. A calculating device according to claim 2, wherein the first and second scales are each divided into a plurality of sub scales which extend parallel to one another and are graduated in terms of surface powers in relation to blanks of different diameters.

4. A calculating device according to claim 2, wherein the surface power indicator means and thickness indicator means comprise a cursor which is slidable relative to all the scales and includes a reference line extending transversely thereto so as to facilitate alignment of selected values of front and back surface powers on the first and second scales and to facilitate reading of the third scale when the cursor is appropriately positioned.

5. A calculating device according to claim 4, wherein the cursor carries additional read-out marker means spaced from the reference line by distances which correspond to the appropriate finishing allowance as referred to herein.

6. A calculating device according to claim 2 wherein a fourth scale graduated in terms of prism compensation is provided associated with the second scale and fixed in position relative thereto.

7. A calculating device according to claim 6, wherein the read-out marker serves as the zero point of the prism compensation scale which is so graduated relative to the third scale as to enable a reading on the third scale to be increased by the extent required by a selected prism factor on the fourth scale.

8. A calculating device according to claim 2, wherein two supplementary scales are provided, each being graduated identically in terms of front surface power, one of these supplementary scales being fixed in position relative to the first scale and the other supplementary scale being slidable relative to the first scale, the

arrangement being such that the slidable supplementary scale can be set in a position which is determined by the relative positions of the first and second scales in such a way that the corrected value of front surface power appears on the fixed supplementary scale against the selected value of the front surface power as indicated on the slidable supplementary scale.

9. A calculating device according to claim 1, wherein the surface power scale means comprises a disc having at least one scale thereon graduated in terms of lens surface powers and wherein the surface power indicator means comprises a pair of cursors rotatable relative to the disc about the axis of the disc, each cursor being capable of indicating a value of lens surface power on the disc, and wherein the thickness scale means comprises a second disc mounted co-axially with the first disc for rotation relative thereto and wherein the thickness indicator means comprises one of the said two cursors.

10. A calculating device according to claim 9, wherein a plurality of scales are provided on the first disc, each scale being graduated in terms of surface powers and being of part-circular form, he scales being concentric with the disc and each scale relating to a blank of a different diameter.

11. A calculating device according claim 9, wherein a scale means graduated in terms of prism compensation is associated with the first disc for causing the thickness of the blank indicated on the thickness scale means to be corrected by the extent required by any selected prism factor, when one of the said two cursors is set to the selected value of prism compensation.

12. A calculating device according to claim 9, wherein a supplementary scale graduated in terms of front surface power and comprising a third disc mounted co-axially with said first and second discs is provided, an indicating pointer being provided for the third disc and said pointer being moved proportionately to the movement of one of the said pair of cursors, the arrangement being such that the nominal front surface power as indicated on the third disc can be set in position relative to the other components of the device in such a way that, when the thickness of the blank is indicated on the thickness scale means, the compensated front surface power is indicated by the indicating pointer on the third scale. 

1. A calculating device for determining the thickness of a blank required to produce a lens, the device comprising surface power scale means graduated in terms of lens surface powers ; surface power indicator means for indicating a selected value of lens power for the front and for the back surface of the lens ; thickness scale means graduated in terms of thickness and movable relative to said first scale means; and thickness indicator means for indicating values of thickness of the thickness scale means ; means being operatively associated with the surface power scale means and the thickness indicator means for ensuring that, for selected values of front and back surface power for the lens, the appropriate thickness of the blank is indicated on the thickness scale means by the thickness indicator means.
 2. A calculating device according to claim 1, wherein the surface power scale means comprises a first scale graduated n terms of front surface powers, a second scale movable relatively to the first scale and graduated in terms of back surface powers, so that any chosen value of back surface power can be aligned with any chosen value of front surface power, and wherein said thickness scale means comprises a third scale disposed adjacent to the second scale and graduated in terms of lens thickness and fixed in position relative to said first scale, read-out marker means being provided iN fixed position relative to the second scale for indicating on the third scale the appropriate thickness of the blank, when a selected value of front surface power on the first scale is aligned with a selected value of back surface power on the second scale.
 3. A calculating device according to claim 2, wherein the first and second scales are each divided into a plurality of sub scales which extend parallel to one another and are graduated in terms of surface powers in relation to blanks of different diameters.
 4. A calculating device according to claim 2, wherein the surface power indicator means and thickness indicator means comprise a cursor which is slidable relative to all the scales and includes a reference line extending transversely thereto so as to facilitate alignment of selected values of front and back surface powers on the first and second scales and to facilitate reading of the third scale when the cursor is appropriately positioned.
 5. A calculating device according to claim 4, wherein the cursor carries additional read-out marker means spaced from the reference line by distances which correspond to the appropriate finishing allowance as referred to herein.
 6. A calculating device according to claim 2 wherein a fourth scale graduated in terms of prism compensation is provided associated with the second scale and fixed in position relative thereto.
 7. A calculating device according to claim 6, wherein the read-out marker serves as the zero point of the prism compensation scale which is so graduated relative to the third scale as to enable a reading on the third scale to be increased by the extent required by a selected prism factor on the fourth scale.
 8. A calculating device according to claim 2, wherein two supplementary scales are provided, each being graduated identically in terms of front surface power, one of these supplementary scales being fixed in position relative to the first scale and the other supplementary scale being slidable relative to the first scale, the arrangement being such that the slidable supplementary scale can be set in a position which is determined by the relative positions of the first and second scales in such a way that the corrected value of front surface power appears on the fixed supplementary scale against the selected value of the front surface power as indicated on the slidable supplementary scale.
 9. A calculating device according to claim 1, wherein the surface power scale means comprises a disc having at least one scale thereon graduated in terms of lens surface powers and wherein the surface power indicator means comprises a pair of cursors rotatable relative to the disc about the axis of the disc, each cursor being capable of indicating a value of lens surface power on the disc, and wherein the thickness scale means comprises a second disc mounted co-axially with the first disc for rotation relative thereto and wherein the thickness indicator means comprises one of the said two cursors.
 10. A calculating device according to claim 9, wherein a plurality of scales are provided on the first disc, each scale being graduated in terms of surface powers and being of part-circular form, he scales being concentric with the disc and each scale relating to a blank of a different diameter.
 11. A calculating device according claim 9, wherein a scale means graduated in terms of prism compensation is associated with the first disc for causing the thickness of the blank indicated on the thickness scale means to be corrected by the extent required by any selected prism factor, when one of the said two cursors is set to the selected value of prism compensation.
 12. A calculating device according to claim 9, wherein a supplementary scale graduated in terms of front surface power and comprising a third disc mounted co-axially with said first and second discs is provided, an indicating pointer being provided for the third disc and said pointer being moved proportionately to the movement of one of the said paiR of cursors, the arrangement being such that the nominal front surface power as indicated on the third disc can be set in position relative to the other components of the device in such a way that, when the thickness of the blank is indicated on the thickness scale means, the compensated front surface power is indicated by the indicating pointer on the third scale. 